1. Field of the Invention
The present invention relates to a generator equipped with a dual voltage inverter having two inverter system, and a generator apparatus having these generators connected in parallel.
2. Description of Related Art
There has been known a generator equipped with an inverter for converting a direct current (DC) into an alternating current (AC) of a desired frequency. Further, there has also been known a generator having two inverters connected in parallel. For example, in Japanese Patent Application Laid-Open No. 8-205543, there is a description of an apparatus for an inverter operation capable of stable parallel operation of two inverters.
It has been desired to obtain two-level output voltages in the above-described generator having one or more inverters. In a generator having an inverter (s), it is possible to obtain two kinds (two levels) of AC output voltages, such as, for example, 120 V and 240 V, by switching an output of a rectification device, that is, a DC voltage input to the inverter, at two levels.
However, in the case of switching over a DC voltage between two levels as described above, it is necessary to make the dielectric strength of the inverter correspond to a higher-voltage side and to make the output current capacity correspond to a higher-current side, respectively. Therefore, it is necessary to increase the size of an inverter circuit and to increase the size of what is called a power element such as an electrolytic capacitor, a choke coil or the like.
There has also been a desire to obtain many kinds of output power, other than changing over an output voltage at two levels or changing over an output power into two levels. However, at present, it is not possible to meet these requirements in an efficient manner by using a compact apparatus.
It is considered possible to obtain two-level AC output voltages by operating two inverters in series or in parallel. FIG. 13 is a connection diagram of a generator having two inverters connected in series. As shown in FIG. 13, it is possible to obtain two-level output voltages of 120 V and 240 V by serially connecting two inverters A and B capable of outputting an AC voltage of 120 V based on a power generated by a generator G. According to the kind of serial operation, however, the balance between the outputs of the two inverters A and B may be broken in some cases depending on the load taken by the inverters.
Further, there is a case where a control method is employed for adjusting a rotational speed of an engine to an optimum value according to the magnitude of a load when a generator is driven by an engine (hereinafter, "eco-throttle control" method). When an electronic governor is set in only one inverter to carry out the eco-throttle control by the CPU of said one inverter and when a heavy load is connected to the other inverter not provided with the electronic governor, for example, there is a problem that it is not possible to secure a sufficient level of an output voltage because the two inverters are out of balance.
In the case of connecting two inverter systems as described above, it is necessary to provide a ground fault interrupter for each series. FIG. 14 is a circuit diagram showing a structure of a generator equipped with a conventional ground fault interrupter. Referring to the drawing, a ground fault interrupter 110 is provided at the output side of a generator 100, and a plug socket 120 for taking out power is connected through the ground fault interrupter 110. The ground fault interrupter 110 is comprised of a detecting section 121 and an operating section 122. The detecting section 121 comprises of a current transformer (ZCT) 121a and an oscillation coil 121b provided on an output line of the generator 100, and a controller 121c for energizing a relay coil 122a of the operating section 122 based on detected signals. A contact point 122b of the operating section 122 is "normally closed", and it is "opened" when the relay coil 122a is energized by the controller 121c.
As the ground fault interrupter 110 is manufactured and marketed as a single integrated unit, it is usually large in size and expensive. When the interrupter is externally installed on the generator, it leads to an increase in the overall size of the generator.